External beam radiotherapy (RT) devices generally include a linear electron beam accelerator which is mounted on a gantry, which can rotate about an axis approximately parallel to the patient lying on the patient couch. The patient is treated using either an electron beam or an X-Ray beam produced from the original electron beam. The electron or X-Ray beam is focused at a target by the combination of the use of a collimator and the rotation of the beam. The patient is placed on a couch that can be positioned such that the target lesion can be located in the plane of the radiation beam as the gantry rotates.
The objective of the radiation therapy is to target the lesion with a high dose of radiation with minimal impact on all the surrounding normal tissue. An initial treatment planning procedure is performed prior to external beam RT delivery to localize the tumour and other critical structures surrounding the tumour. This planning procedure typically involves CT imaging to identify these structures. Based on the segmented tumour and surrounding tissue structures, a set of beam orientations and collimator settings are developed through an iterative process to determine the optimal dose distribution pattern that maximizes dose to the tumour whilst minimizing dose to surrounding critical avoidance structures.
MRI is currently the optimal modality for tumour localization based on the higher soft-tissue contrast, compared with CT, and can be incorporated into the treatment planning workflow. Although MRI provides good location of the tumour for treatment planning purposes, these treatment planning images are normally collected several days prior to treatment, and, as such, may not be completely representative of tumour location on the day of treatment. To address this limitation, oncologists tend to increase the target volume ensure all the tumor tissue receives the maximum dose. The expectation is that all cells in the targeted region will receive the required RT treatment dose, and that this increased treatment target volume will lessen the impact of errors between treatment planning dose distribution, and the dose delivered to the actual region of the lesion. However, this increased treatment margin also produces collateral tissue damage that may have a significant impact of the quality of life of the patient and increase the possibility of secondary RT-induced cancer.
To mitigate the need for increased treatment margins, clinicians have employed a method referred to as image-guided external beam radiation therapy, in which an image is acquired immediately prior to RT treatment delivery. One such available solution involves completely integrating the MRI system with a linear accelerator to enable real-time imaging of the tumour during RT treatment. However, this design is complex, expensive and may involve serious compromises on the functional performance of both the MRI and linear accelerator.
The treatment planning images are typically collected days prior to the actual fractionated treatment delivery that can occur over the course of several weeks. As such, the position of the tumour in the treatment imaging plans may not be representative of the actual lesion position on each day of treatment. By incorporating image guidance immediately before each treatment session, it is possible to determine the exact position of the lesion within each treatment session. Acquiring MR images immediately before RT treatment would identify the exact lesion location, and define the correct gantry positions for conformal radiation delivery.
To integrate pre-treatment MR images into the RT treatment workflow requires a mapping of the MRI coordinate space to the RT system coordinate space, to ensure correct alignment of the linear accelerator gantry for treatment. The RT system coordinate space is defined by the gantry/room mounted x-ray/cone beam CT (CBCT) system that is typically used in conventional image guided-radiation therapy. One solution would involve registering the pre-treatment MRI and x-ray images to determine the relative coordinate transformation required to align “MRI space” and “RT space”, using manual registration of fiducial markers on the surface, or inserted into, the patient. Typically this involves an operator identifying multiple control points in the MRI space and also identifying the same representative points in the RT unit's X-ray images. Although simplistic, this is a manual, time-consuming, and error-prone technique. Moreover, it is possible that the fiducial markers might move, particularly if affixed to the skin of the patient. Alternatively, patient registration can be performed using an image-based anatomical registration method that maps the two coordinates systems using specific anatomical features. This alternative method is based solely on patient anatomy and involves registering two image datasets with considerably different image contrasts, and is prone to registration errors.
A radiotherapy device generally includes a linear electron beam accelerator which is mounted on a gantry and which can rotate about an axis which is approximately parallel to the patient lying on the patient couch. The patient is treated using either an electron, γ- or X-Ray beam produced from the original electron beam. The beam is focused at a target by the combination of the use of a collimator and the rotation of the beam. The patient is placed on a couch which can be positioned such that the target lesion can be located in the plane of the electron beam as the gantry rotates. This patient couch is designed to adjust the position of the patient to align the targeting exactly at the isocentre of the RT system using up to six degrees of motion (x, y, z, roll, pitch, and yaw). The current couch designs employed by several manufacturers employ a cantilevered couchtop that enables a sufficient range of motion to treat disease sites throughout the entire body.
Bucholz et al. discloses a method to combine proton beam therapy with an MRI system in U.S. Pat. No. 6,862,469. This method only discusses proton therapy, describes a stationary MRI, in which the beam is sent through a gap in the magnet. This application suggests mat shielding methods can be used to remove magnetic and RF interference, although this is only briefly mentioned.
Dempsey discloses a method to deliver RT using cobalt-60 as the source of ionizing radiation with a stationary open MRI system in U.S. Patent Application Publication No. 2005/0197564 device and a process for performing high temporal- and spatial-resolution MR imaging of the anatomy of a patient during intensity modulated radiation therapy (IMRT) to directly measure and control the highly conformal ionizing radiation dose delivered to the patient for the treatment of diseases caused by proliferative tissue disorders. This invention combines the technologies of open MRI, multileaf-collimator or compensating filter-based IMRT delivery, and cobalt teletherapy into a single co-registered and gantry mounted system.
Carlone discloses a method to combine MRI and a radiation therapy system in WO/2009/155700, entitled Radiation Therapy system. This method was developed in Alberta and describes an approach that exposes the linear accelerator to the magnetic field, and uses the magnet forces to direct the particles along the central axis.
Lagendijk discloses a method to combine MRI and radiation therapy using a global coordinate system in WO/2003/008986, entitled MRI in guided radiotherapy and position verification. In this system the MRI is actively shielded to prevent the static magnetic fields from interfering with the linear accelerator operation.
Orbital Therapy discloses in U.S. Pat. No. 7,758,241 a self-shielded radiotherapy device, that does not require a traditional bunker for operation.
Other patents describing prior art include:    U.S. Pat. No. 6,198,957—Radiotherapy Machine including Magnetic Resonance imaging system    U.S. Pat. No. 6,366,798—Radiotherapy machine including Magnetic Resonance Imaging system.    U.S. Pat. No. 6,725,078—System combining proton beam irradiation and magnetic resonance imaging.    U.S. Pat. No. 5,402,783—Method of Minimizing distortion to radiation isodose.    U.S. Pat. No. 6,419,680—CT and MRI visible index markers for stereotactic localization. In this patent application, the inventors claim that skin-based localizer markers can be used for stereotactic localization in both MRI and CT.    WO 03/008986 A2 MRI in guided radiotherapy and position verification Submitted by Utrecht, this patent describes incorporating an independent world coordinate isocentre calibration system consisting of fiducial table MR-markers and an independent table position verification system.    IMRIS also has also filed PCT Application PCT/CA2010/000422 filed Mar. 29, 2010 published 7 Oct. 2010 under publication no. WO 2010/111772 for a patient support system for integrating X-ray imaging with MR, in Support Component for Use in Imaging by Magnetic Resonance and X-ray. This application describes a support structure that is both MR compatible and radiolucent.    In U.S. Pat. No. 5,778,047 (Mansfield) issued Jul. 7, 1998 Varian discloses patient couch-top in entitled Radiotherapy Couch Top; however, materials in the design include carbon fiber, and therefore not MR compatible. The design includes bearings for longitudinal motion that enables panels in the couch top to be inserted and removed.    In U.S. Pat. No. 3,720,817 (Dinwiddie) issued Mar. 13, 1973 Varian also has a patent on the overall RT system, entitled Automated Radiation Therapy Machine. In this patent, the patient couch is identified and described.
The disclosure of all of the above cited references is hereby incorporated herein by reference or may be referred to for further details of components and methods not specifically set out herein.